1. Field of Invention
The present invention relates to a D flip-flop. More particularly, the present invention relates to a dynamic floating input D flip-flop (DFIDFF).
2. Description of Related Art
Recently, with the development of the process, handheld electronic products have become the necessary tools in life. For the handheld electronic products, it is an important topic on the product design that the power consumption is saved to increase the lifetime of the cell and to prolong the service time of the handheld electronic product. Viewing from the power consumption formula P=αcV2f, it is the most effective method to reduce the operating voltage. P is the power consumption, α is the activity coefficient, c is the capacitance value, v is the voltage value, and f is the operating frequency. However, in practice, it is necessary to correspondingly reduce the operating frequency to reduce the operating voltage. Therefore, it is an important topic to operate at high speed under the low voltage.
For example, in a wireless communication system, the phase-locked loop (PLL) is one of the most important basic blocks. FIG. 1A is a block diagram of a common PLL. Referring to FIG. 1A, the PLL includes a phase detector 10, a low pass filter 120, a voltage controlled oscillator 130, and a frequency divider 140. The working frequencies of the voltage controlled oscillator 130 and the frequency divider 140 limit the whole operating speed, therefore, if the voltage controlled oscillator 130 and the frequency divider 140 are effectively improved, the performance of the whole PLL is increased.
The operation formula of the frequency divider 140 is fout=fin/2M, wherein M is the number of the D flip-flops connected in series. FIG. 1B is a circuit diagram of a basic 1/16 frequency divider 140. Referring to FIGS. 1A and 1B, because a first flip-flop 141 receives the clock signal generated by the voltage controlled oscillator 130, the operating speed of the frequency divider 140 is limited by the speed the first D flip-flop 141 can achieve. FIG. 1C is a circuit diagram of a conventional transmission gate D flip-flop (TGFF). The D flip-flop includes transmission gates 151, 152, 155, and 156, and NOT gates 153, 154, 157, and 158. Under the condition of the CMOS process of 0.13 μm and the supply voltage of 0.5 V, the highest operating frequency of the 1/16 frequency divider (as shown in FIG. 1B) formed by the conventional D flip-flop as shown in FIG. 1C is 285 MHz, and the power consumption is 1.17 μW.
In view of the above, the 1/16 frequency dividing circuit formed by the conventional TGFFs cannot achieve the high speed operation. Therefore, in “A True Single-Phase-Clock Dynamic CMOS Circuit Technique” published in the Journal of Solid-State Circuit, vol. sc-22, NO. 5, page 899-901, October 1987 by the Institute of Electrical and Electronic Engineers (IEEE) (as shown in FIG. 2), and “New single-clock CMOS latches and flip-flops with improved speed and power savings” published in the Journal of Solid-State Circuit, vol. sc-32, NO. 1, page 62-69, 1997 (as shown in FIG. 3), a true signal phase clock (TSPC) D flip-flop circuit is provided, wherein the dynamic operating is used to achieve the high speed characteristics. However, because the number the of metal oxide semiconductor (MOS) transistors connected in series in the conventional art is larger (as shown in FIG. 2, the number of the MOS transistors connected in series is four at most), the whole speed is affected when operating in low voltage. Therefore, in the conventional art, under the condition of the CMOS process of 0.13 μm and the rigor condition (i.e. 0.45_SS—125° C.), the operating frequency of the D flip-flop of FIG. 2 may be up to 400 MHz, with the power consumption of 2.8 μW. The D flip-flop of FIG. 3 may operate at 454 MHz, with the power consumption of 4.2 μW.
In addition, in the conventional art, the number of the MOS transistors connected in series may be reduced to achieve the high speed characteristics under the low voltage. For example, in “A 1.6-GHz dual modulus prescaler using the Extended True-Single-Phase-Clock CMOS circuit technique (E-TSPC)” published in the Journal of Solid-State Circuit, vol. sc-34, page 97-102, January 1999 by IEEE (as shown in FIG. 4), a high speed D flip-flop of E-TSPC architecture is provided. The dynamic D flip-flop circuit is realized by six MOS transistors divided into three stages. In the conventional art, a certain ratio relation exists between each stage, so it must be quite careful in design. Under the condition of the CMOS process technique of 0.13 μm and the working voltage of 0.5 V, the conventional art is used to realize the 1/16 frequency diving circuit, and the quickest operating frequency thereof may be up to 1 GHz. However, because the circuit connecting manner may generate large short circuit current and dc current, a lot of power consumption may be resulted. Particularly in the low speed, the short circuit current and the dc current take more than half of the total power consumption. The D flip flop is made to be a divided-by two circuit (as shown in FIG. 4A), and it can be seen that during operating, the dashed line region is the position generating the short circuit current and the dc current.
In addition, in “New Dynamic Flip-Flop for High-Speed Dual-Modulus Prescaler” published in the Journal of Solid-State Circuit, vol. sc-33, No. 10, page 1568-1571, 1998 by IEEE, another dynamic D flip-flop is provided, as shown in FIG. 5. The flip-flop is similar to the flip-flop architecture of FIG. 4, and the difference is that three MOSs connected in series are used in the middle stage, the output stages also have the ratio relation, so it is necessary to design carefully. In the D flip-flop, because some paths of the short circuit current and the dc current are reduced, the power lost is reduced slightly. Under the condition of the CMOS process of 0.13 μm and the working voltage of 0.5 V, the quickest frequency of the flip-flop in FIG. 5 may be more than 1 GHz, with the power consumption of 9.26 μW.
Although under the low voltage, the operating frequency of the conventional D flip-flop circuits as shown in FIGS. 4 and 5 may be more than 500 MHz, because of the connecting manner of the flip-flop circuit, it is much possible to generate the short circuit current and the dc current, thereby consuming a lot of power.